<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body >

	  
	  <form name="Dem">
	  
	  <input type="text" size="8" name="dongho" readonly>
	  </form> 

<script> 
 var phut = 0
 var giay = 10
function DemNguoc(){ 
 if(giay<=0){
    phut -=1
    giay=60
 }
 if (phut<=-1){ 
    giay=0
    phut+=1 
	HetGio()
 } 
 else {
    giay-=1 
    document.Dem.dongho.value=phut+":"+giay 
    setTimeout("DemNguoc()",1000) }
}

function HetGio(){
	alert("Het Gio, goi submmit ");
}

DemNguoc() 

</script> 

</body>
</html>
